Process and device for low-temperature separation of air

ABSTRACT

The process and the device are used for low-temperature separation of air with a distilling-column system for nitrogen-oxygen separation ( 20 ), which has at least one separation column ( 21, 22 ). A main air stream ( 1, 5 ) is compressed in an air compressor ( 2 ) and purified in a purification device ( 4 ). A first air stream ( 7 ) and a second air stream ( 8 ) are diverted from the main air stream ( 5 ). The first air stream ( 7 ) is further compressed in two secondary compressors ( 10, 13 ) that are connected in series. The further compressed first air stream ( 15 ) is cooled by indirect heat exchange ( 16 ), and at least partially liquefied or pseudo-liquefied, and then introduced into the distilling-column system for nitrogen-oxygen separation ( 20 ). The second air stream ( 8 ) is cooled by indirect heat exchange ( 16 ) and then, divided into two partial streams ( 24, 27 ), is actively depressurized in two expanders ( 25, 28 ), whereby the two expanders have essentially the same inlet pressure. The actively depressurized partial streams ( 26, 29 ) of the second air stream are introduced ( 30, 129 ) at least in part into the distilling-column system for nitrogen-oxygen separation ( 20 ). The mechanical energy that is produced in the active depressurization ( 25, 28 ) of the second air stream is used at least partially in the driving of the two secondary compressors ( 10, 13 ) that are connected in series. A liquid product stream ( 31 ) is removed from the distilling-column system for nitrogen-oxygen separation ( 20 ), brought to an elevated pressure in the liquid state ( 32 ), and evaporated or pseudo-evaporated under this elevated pressure by indirect heat exchange ( 16 ) with the first air stream ( 15 ), and finally removed as a gaseous product stream ( 34 ). Both secondary compressors ( 10, 13 ) are operated with an inlet temperature that is higher than 250 K, in particular higher than 270 K.

The invention relates to a process for low-temperature separation of air according to the introductory clause of Claim 1.

Processes and devices for low-temperature separation of air are known from, for example, Hausen/Linde, Tieftemperaturtechnik [Low-Temperature Technology], 2^(nd) Edition 1985, Chapter 4 (pages 281 to 337)

The distilling-column system of the invention can be designed as a one-column system for nitrogen-oxygen separation, as a two-column system (for example as a standard Linde double-column system), or else as a three-column or multiple-column system. In addition to the columns for nitrogen-oxygen separation, additional devices can be provided for recovering other air components, in particular noble gases, for example an argon- or a krypton-xenon recovery.

The invention relates in particular to a process in which at least one gaseous compressed product is recovered by a liquid product stream being removed from the distilling-column system for nitrogen-oxygen separation, brought to an elevated pressure in the liquid state, and evaporated under this elevated pressure by indirect heat exchange or pseudo-evaporated (at supercritical pressure). Such internal compression processes are known from, for example, DE 830805, DE 901542 (a U.S. Pat. No. 2,712,738/U.S. Pat. No. 2,784,572), DE 952908, DE 1103363 (=U.S. Pat. No. 3,083,544), DE 1112997 (=U.S. Pat. No. 3,214,925), DE 1124529, DE 1117616 (=U.S. Pat. No. 3,280,574), DE 1226616 (=U.S. Pat. No. 3,216,206), DE 1229561 (=U.S. Pat. No. 3,222,878), DE 1199293, DE 1187248 (=U.S. Pat. No. 3,371,496), DE 1235347, DE 1258882 (=U.S. Pat. No. 3,426,543), DE 1263037 (=U.S. Pat. No. 3,401,531), DE 1501722 (=U.S. Pat. No. 3,416,323), DE 1501723 ( U.S. Pat. No. 3,500,651), DE 2535132 (=U.S. Pat. No. 4,279,631), DE 2646690, EP 93448 B1 (=U.S. Pat. No. 4,555,256), EP 384483 B1 (=U.S. Pat. No. 5,036,672), EP 505812 B1 (=U.S. Pat. No. 5,263,328), EP 716280 B1 (=U.S. Pat. No. 5,644,934), EP 842385 B1 (=U.S. Pat. No. 5,953,937), EP 758733 B1 (=U.S. Pat. No. 5,845,517), EP 895045 B1 (=U.S. Pat. No. 6,038,885), DE 19803437 A1, EP 949471 B1 (=U.S. Pat. No. 6,185,960 B1), EP 955509 A1 (=U.S. Pat. No. 6,196,022 B1), EP 1031804 A1 (=U.S. Pat. No. 6,314,755), DE 19909744 A1, EP 1067345 A1 (=U.S. Pat. No. 6,336,345), EP 1074805 A1 (=U.S. Pat. No. 6,332,337), DE 19954593 A1, EP 1134525 A1 (=U.S. Pat. No. 6,477,860), DE 10013073 A1, EP 1139046 A1, EP 1146301 A1, EP 1150082 A1, EP 1213552 A1, DE 10115258 A1, EP 1284404 A1 (=US 2003051504 A 1), EP 1308680 A1 (=U.S. Pat. No. 6,612,129 B2), DE 10213212 A1, DE 10213211 A1, EP 1357342 A1 or DE 10238282 A1. A process of the above-mentioned type is known from WO 2004/099690.

The object of the invention is to structure such a process and a corresponding device in an especially advantageous manner economically.

This object is achieved in that both secondary compressors are operated at an inlet temperature that is higher than 250 K, in particular higher than 270 K.

Both secondary compressors are thus operated under warm conditions. In this respect, well-tested techniques can be used, for example, two identical turbine-booster combinations. In addition, the heat exchanger volume is relatively small, and thus investment costs are saved.

The expanders are preferably designed as turbines. They have “essentially the same inlet pressure,” i.e., their inlet pressures are distinguished, if necessary, by various pressure losses in lines, heat exchanger passages or the like. The inlet temperatures of the two expanders are the same or different and rest on one or two intermediate levels between the hot and the cold ends of the main heat exchanger.

The invention can be applied to processes with exactly two air streams and the division of the second air stream into exactly two partial streams. As an alternative, in the invention, one or more additional air streams and/or one or more additional partial streams can be used. For example, it is possible to use three or more expanders. These can be—but do not have to be—connected in parallel on the inlet side.

The two or more expanders of the invention can also be connected in parallel on the outlet side, i.e., they can have essentially the same outlet pressure and essentially the same outlet temperature. As an alternative to this, at least two of the expanders that are connected in parallel on the inlet side have different pressures.

The transfer of the mechanical energy from the active depressurization is preferably produced by a direct mechanical coupling of a first of two expanders that are connected in parallel to the first of two secondary compressors that are connected in series and by a direct mechanical coupling of the second of two expanders to the second of two secondary compressors.

The application of the invention to a two-column or multiple-column system, which has at least one high-pressure column and one low-pressure column, is especially advantageous, whereby the operating pressure of the low-pressure column is less than the operating pressure of the high-pressure column.

Preferably, a first of the two partial streams is introduced into the high-pressure column downstream from its active depressurization. The outlet pressure of the corresponding expansion turbine is in this case approximately at the level of the operating pressure of the high-pressure column.

The second of the two partial streams can then also be reduced in pressure to approximately high-pressure column pressure and can be introduced, for example, together with the first into the high-pressure column.

As an alternative to this, the second of the two partial streams of the second air stream is introduced at least partially into the low-pressure column. It is thus possible to set the outlet pressure of the corresponding expansion turbine at a lower value and under decompression to do more work owing to the elevated pressure ratio and thus to produce more cold.

In a three-column or multiple-column system, if i.e., the distilling-column system for nitrogen-oxygen separation has a high-pressure column, a medium-pressure column and a low-pressure column, which are operated under various pressures, the first partial stream can be introduced at least in part into the high-pressure column and the second partial stream can be introduced at least in part into the medium-pressure column and/or the low-pressure column.

In many cases, it is advantageous when the first air stream upstream from the first secondary compressor and the first air stream downstream from the second secondary compressor are brought into indirect heat exchange. In this connection, the first air stream is heated before the first secondary compressor and cooled again behind the second secondary compressor. Thus, the first air stream enters the main heat exchanger at a temperature that is lower than that behind the second secondary compressor or behind its secondary condenser. Typically, this temperature difference is 1 to 10 K, preferably 2 to 5 K. Thus, the product streams can be drawn off from the main heat exchanger under lower temperatures, which has advantageous effects for the pre-cooling of air and for the cooling of the molecular sieve for the air purification.

As an alternative or in addition, standard intermediate condensers or secondary condensers can be used that remove the compression that accumulates in the secondary compressors by indirect heat exchange with an external coolant, for example with cooling water. In this connection, one or two secondary condensers can be used by having only the first secondary compressor, only the second secondary compressor or both secondary compressors per secondary condenser. In principle, it is also possible to eliminate the secondary condenser and the above-described indirect heat exchange completely. In general, however, at least the first secondary compressor has a secondary condenser (intermediate condenser).

In addition, the invention relates to a device for low-temperature separation of air according to claim 9.

The invention as well as additional details of the invention are explained in more detail below based on the embodiments that are depicted diagrammatically in the drawings. In this connection:

FIG. 1 shows a first embodiment of the invention, and

FIG. 2 shows a second embodiment with a cold compressor.

In the embodiment of FIG. 1, atmospheric air is suctioned off as a main air stream via line 1 from an air compressor 2, brought there to a first pressure of 10 to 30 bar, preferably approximately 19 bar, cooled in a pre-cooling stage 3 to approximately ambient temperature and fed to an adsorptive air purification stage 4. The purified main air stream 5 is divided at 6 into a first air stream 7 and a second air stream 8.

The first air stream is heated in a booster-heat exchanger 9 to approximately cooling-water temperature and further compressed in a first secondary compressor 10 to an intermediate pressure of 15 to 60 bar, preferably approximately 25 bar. Then, the compression heat is removed at least partially in a first secondary condenser 1. The first air stream 12 is then still further compressed in a second secondary compressor 13 to a final pressure of 22 to 90 bar, preferably approximately 40 bar, and then heated in a second secondary condenser 14 and the booster-heat exchanger 9 to slightly above cooling-water temperature. Under this final pressure, the first air stream 15 enters into a main heat exchanger 16 and is cooled there and liquefied, or (at supercritical pressure) pseudo-liquefied. The first air stream 17, which is cold, is reduced to a pressure of 4 to 10 bar, preferably approximately 6 bar (in the example in a butterfly valve 18), and introduced under this pressure in at least partially liquid state via line 19 into the high-pressure column 21 of a distilling-column system for nitrogen-oxygen separation 20, which in addition has a low-pressure column 22, a condenser-evaporator, not shown, and a subcooling countercurrent device 23.

The second air stream 8 is not further compressed. It is introduced under the first pressure into the main heat exchanger 16 and is cooled there to an intermediate temperature of 125 to 200 K, preferably approximately 140 K. The second air stream is divided at this intermediate temperature into two partial streams 24, 27, and is subjected to the active depressurization in two turbines 25, 28 that are connected in parallel, which both reduce the pressure to approximately the operating pressure of the high-pressure column 21. The two depressurized partial streams 26, 29 are purified again and introduced essentially in the gas state into the high-pressure column 21 via line 30.

Oxygen 31 is drawn off directly or via a liquid tank as a “liquid product stream” from the low-pressure column 22 of the distilling-column system for nitrogen-oxygen separation 20 and is brought by a pump 32 in the liquid state to an elevated pressure of 4 to 70 bar, preferably approximately 40 bar. Under this elevated pressure, the liquid or supercritical oxygen 33 is evaporated or pseudo-evaporated in the main heat exchanger 16 by indirect heat exchange with the first air stream and heated to approximately ambient temperature. The oxygen is ultimately released as a gaseous product stream 34. One or more additional product or residual streams 35 can be drawn off via the main heat exchanger from the distilling-column system for nitrogen-oxygen separation 20. In addition or as an alternative to the internal compression of oxygen that is shown in the drawings, nitrogen—for example from the main condenser or from the high-pressure column of the distilling-column system for nitrogen-oxygen separation 20—can also be compressed internally in an analogous way.

In the embodiment of FIG. 1, the first turbine 25 and the first secondary compressor 10 as well as the second turbine 28 and the second secondary compressor 13 are coupled mechanically in pairs in each case via a common shaft.

The booster-heat exchanger 9 and the secondary condenser 14 are optional. They can be omitted individually or in their entirety.

FIG. 2 shows an embodiment that contains two variants relative to the process of FIG. 1, which can both be applied independently of one another. The same or comparable process steps carry the same reference numbers as in FIG. 1.

The first variant relates to the outlet pressure of the second turbine 28. The latter reduces the pressure here to 1.2 to 4 bar, preferably approximately 1.4 bar, i.e., approximately the operating pressure of the low-pressure column 22. The depressurized lower partial stream 129 is then blown into the low-pressure column. The inlet pressures of the two turbines 25, 28 are, however, still the same; the inlet temperatures can be the same or different.

In a second variant, the second secondary compressor 113 is designed as a cold compressor. The first air stream 12 a, 12 b, 12 c is therefore already introduced under intermediate pressure into the main heat exchanger 16 and is removed again from the main heat exchanger 16 at a second intermediate temperature of 120 to 180 K, preferably approximately 48 K. This second intermediate temperature can be less than or equal to the inlet temperature of the turbines 25, 28, preferably it is—contrary to the depiction in the drawing—higher. Downstream from the cold compression stage 113, the second air stream 115 is introduced at a third intermediate temperature, which is higher than the turbine inlet temperature, and 140 to 220 K, preferably approximately 180 K, is again introduced into the main heat exchanger 16.

Contrary to the embodiment in FIG. 2, the second air stream can be conveyed upstream from the cold secondary compressor 113 up to the cold end of the main heat exchanger 16, and in this case, it is at least partially liquefied. It is then slightly throttled, again introduced into the cold end of the main heat exchanger, again evaporated, and finally heated to the inlet temperature of the compressor 113, as it is explained in detail in, for example, EP 1067345 B1. 

1. Process for low-temperature separation of air with a distilling-column system for nitrogen-oxygen separation (20), which has at least one separation column (21, 22), in which A main air stream (1, 5) is compressed in an air compressor (2) and purified in a purification device (4), A first air stream (7) and a second air stream (8) are diverted from the main air stream (5), The first air stream (7) is further compressed in two secondary compressors (10, 13) that are connected in series, The further compressed first air stream (15) is cooled by indirect heat exchange (16) and at least partially liquefied or pseudo-liquefied, and then introduced into the distilling-column system for nitrogen-oxygen separation (20), The second air stream (8) is cooled by indirect heat exchange (16) and then, divided into two partial streams (24, 27), is actively depressurized in two expanders (25, 28), whereby the two expanders have essentially the same inlet pressure, The actively depressurized partial streams (26, 29) of the second air stream are introduced (30, 129) at least in part into the distilling-column system for nitrogen-oxygen separation (20), The mechanical energy that is produced in the active depressurization (25, 28) of the second air stream is used at least partially in the driving of the two secondary compressors (10, 13) that are connected in series, A liquid product stream (31) is removed from the distilling-column system for nitrogen-oxygen separation (20), brought to an elevated pressure in the liquid state (32), and evaporated or pseudo-evaporated under this elevated pressure by indirect heat exchange (16) with the first air stream (15), and finally removed as a gaseous product stream (34), characterized in that both secondary compressors (10, 13) are operated at an inlet temperature that is higher than 250 K, in particular higher than 270 K.
 2. Process according to claim 1, wherein the distilling-column system for nitrogen-oxygen separation (20) has a high-pressure column (21) and a low-pressure column (22).
 3. Process according to claim 2, wherein a first (26) of the two partial streams of the second air stream is introduced (30) at least in part into the high-pressure column (21).
 4. Process according to claim 3, wherein the second (29) of the two partial streams of the second air stream is introduced (30) at least in part into the high-pressure column (21).
 5. Process according to claim 2, wherein the second of the two partial streams of the second air stream is introduced (129) at least in part into the low-pressure column (22).
 6. Process according to claim 1, wherein the distilling-column system for nitrogen-oxygen separation has a high-pressure column, a medium-pressure column, and a low-pressure column, whereby the first partial stream is introduced at least in part into the high-pressure column and the second partial stream is introduced at least in part into the medium-pressure column and/or the low-pressure column.
 7. Process according to claim 1, wherein the first air stream upstream from the first secondary compressor and the first air stream downstream from the second secondary compressor are brought together in indirect heat exchange (9).
 8. Process according to claim 1, wherein only the first secondary compressor, only the second secondary compressor, or both secondary compressors in each case have a secondary condenser (11, 14).
 9. Device for low-temperature separation of air for low-temperature separation of air with a distilling-column system for nitrogen-oxygen separation (20), which has at least one separation column (21, 22), with An air compressor (2) for compressing a main air stream (1), Purification device (4) for purifying the compressed main air stream, Means for diverting a first and second air stream (7, 8) from the main air stream (5), Two secondary compressors (10, 13) that are connected in series for further compression of the first air stream (7), Means (16, 20) for cooling and liquefaction or pseudo-liquefaction of the further compressed first air stream (15) by indirect heat exchange and for introduction thereof into the distilling-column system for nitrogen-oxygen separation (20), Means (16) for cooling the second air stream (8) by indirect heat exchange (16) to an intermediate temperature, Two expanders (25, 28) that are connected in parallel on the inlet side for active depressurization of the cooled second air stream in two partial streams (24, 27), Means (26, 29, 30, 129) for introducing the actively depressurized partial streams (26, 29) of the second air stream into the distilling-column system for nitrogen-oxygen separation (20), Means for transfer of the mechanical energy that is produced in the active depressurization (25, 28) of the second air stream to the two secondary compressors (10, 13) that are connected in series, Means (31, 32, 33, 16, 34) for removing a liquid product stream (31) from the distilling-column system for nitrogen-oxygen separation (20), for increasing the pressure of the liquid product stream that is brought to an elevated pressure in the liquid state (32), for evaporation or pseudo-evaporation under this elevated pressure by indirect heat exchange with the first air stream (15), and for drawing off as a gaseous product stream (34), wherein both secondary compressors (10, 13) are connected to means for feeding the first air stream under an inlet temperature that is higher than 250 K, in particular higher than 270 K. 